Defense and prosecution attorneys will both point to the same facts to make their case as Marilyn Lemak's trial gets under way. She is charged in the 1999 deaths of her three young children.

Even her attorneys acknowledge Marilyn Lemak ended the lives of her three young children on a late-winter day nearly three years ago, doping them with prescription drugs and suffocating them with her hands. So her murder trial comes down to this: Was Lemak legally insane, or should she be held criminally responsible for the slayings?

Jury selection in the case concluded earlier today, and opening statements by lawyers on both sides began this afternoon.

Prosecutors plan to seize on evidence they believe demonstrates that the 44-year-old's thinking was sharp the day she killed the children. They'll say Lemak was going through a drawn-out divorce, plotted a crime meant to spite her husband, tried to conceal evidence and then showed remorse. Her ex-husband will be their first witness.

The challenge for defense attorneys will be to use many of the same facts in the service of a different theory: that Lemak's mental state collapsed along with her 14-year marriage to emergency-room physician David Lemak.

Defense attorneys will try to harness the horror of her acts--methodically killing her own children--to demonstrate how sick she was when she sedated and smothered Nicholas, 7, Emily, 6, and Thomas, 3.

Lemak reverted to her maiden name of Morrissey as part of a divorce settlement, but she still is referred to in criminal court as Marilyn Lemak.

If the jury finds her not guilty by reason of insanity, Lemak will likely spend years in a mental institution. A verdict of guilty or guilty but mentally ill could send her to prison for life or to Death Row.

Jurors will be asked to sift through some facts that are so open to interpretation that both sides likely will spin them to support their case. How, for instance, does one interpret Lemak's conversation with a 911 dispatcher, the morning she awoke to find her children dead and her apparent suicide attempt botched?

"My three kids are dead, and I ... I wanted to be dead too but it didn't work," she told the dispatcher.

"How did you do this, what happened?" the dispatcher asked.

"My husband didn't want us anymore," she replied. When told that police and paramedics were coming, Lemak said, "My dogs are barking. The dogs are really nice. ... They're old, and they are real nice. And one little cat. Don't let 'em let the cat run out."

On the master bedroom dresser, police found a picture of David Lemak and his new girlfriend, with a bloody X-acto knife stuck through the chest of his image. Is that evidence of a vengeful wife furious at her estranged husband, or a sign that she had slipped into psychosis?

Other elements of the crime are equally ambiguous. Prosecutors point to the methodical way she killed the children as evidence of her culpability. The same facts convince the defense of her insanity.

Authorities say Emily and Thomas were killed while Nicholas played downstairs. After Lemak came downstairs, "she told [Nicholas] that Emily and Thomas would not be joining them for dinner," Naperville Police Detective Mike Cross said at an inquest into the children's deaths, based on an interview of Lemak at Naperville's Edward Hospital shortly after her arrest.

"She then gave him a bagel [and] spread it with crunchy peanut butter," Cross said, describing how Lemak sprinkled it with crushed tablets of the anti-anxiety tranquilizer Ativan, which physicians had prescribed to the part-time surgical nurse.

DuPage County State's Atty. Joseph Birkett also has pointed to statements Lemak made to police and paramedics as proof that she couldn't wait to get the guilt off her chest. To the defense, authorities exploited the ramblings of a deranged woman.

Focus on scorn

Prosecutors will concentrate on evidence that supports their revenge-of-a-woman-scorned theory.

In the weeks leading up to the killings, authorities have said, Marilyn Lemak began to outwardly display her anger, berating her husband as a liar and a cheat in handwritten notes. After his wife twice filed for divorce and sought to kick him out of the house, David Lemak moved to a rented home down the street and later began dating a nurse at Hinsdale Hospital, where he worked.

Once she discovered that relationship, Marilyn Lemak allegedly made harassing phone calls to the girlfriend. One night before her husband moved out when she suspected he was with the woman, Lemak bolted all the doors, shuttered the windows and ripped out the doorbell, her friends and authorities have said.

Police and prosecutors believe two things triggered the slayings: Lemak saw her husband and girlfriend together March 3, the day before the killings, and David Lemak was to have the children for a court-ordered visit March 5.

Lemak family physician Robert Hubbard testified in a pretrial hearing that Marilyn Lemak told him after the slayings that "she had seen her husband with his girlfriend the prior day and realized she and her children were no longer the most important thing in his life."